Capacitive touch screens and panels are used as a user interface to electronic equipment, e.g., computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch screens and panels provide soft key functionality. In other applications, transparent touch screens overlay a display to allow the user to interact, via touch or proximity, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch screen or panel is activated (controls a signal indicating activation) by a change in capacitance of a capacitive electrode in the touch screen or panel when an object, e.g., a user's finger tip, causes the capacitance of the capacitive electrode to change.
Today's capacitive touch screens and panels come in different varieties, including single-touch and multi-touch. A single-touch screen or panel detects and reports the position of one object in contact or proximity with the touch screen or panel. A multi-touch screen or panel detects the position of one or more objects in simultaneous contact or proximity with the touch screen or panel, and reports or acts upon distinct position information related to each object.
Touch screens and panels used in both single-touch and multi-touch systems may comprise one or more layers, each layer having a plurality of electrodes electrically insulated from each other. In a multi-layer touch sensor, the layers may be fixed in close proximity to and electrically insulated from each other. In any of the one or more layer touch screen and panel constructions, the electrodes (capacitances) may form any type of coordinate system (e.g., polar, etc.). Some touch sensors may utilize an X-Y or grid-like arrangement. Referring to FIG. 1, depicted is a schematic plan view of touch sensor electrodes arranged in an X-Y grid orientation, according to the teachings of this disclosure. For example, in a two-layer touch screen or panel 102, electrodes 104 and 105 are on different layers (substrate 106) and may be arranged orthogonal to each other such that the intersections thereof, referred to hereinafter as nodes 120, between the electrodes 104 and 105 on the different layers define a grid (or other coordinate system). In an alternative, single-layer touch screen, the proximity relationship between one set of electrodes and another set of electrodes may similarly define a grid (or other coordinate system).
Measuring the self capacitance of individual electrodes within the touch screen or panel is one method employed by single-touch systems. For example, using an X-Y grid a touch sensor controller iterates through each of the X-axis and Y-axis electrodes 105 and 104, respectively, selecting one electrode at a time and measuring its capacitance. The position of touch is determined by the proximity of (1) the X-axis electrode 105 experiencing the most significant capacitance change, and (2) the Y-axis electrode 104 experiencing the most significant capacitance change.
Performing self capacitance measurements on all X-axis and Y-axis electrodes provides a reasonably fast system response time. However, it does not support tracking multiple simultaneous (X,Y) coordinates, as required in a multi-touch screen system. For example, in a 16×16 electrode grid, the simultaneous touch by one object at position (1,5) and a second object at position (4,10) leads to four possible touch locations: (1,5), (1,10), (4,5), and (4,10). A self-capacitance system is able to determine that X-axis electrodes 1 and 4 have been touched and that Y-axis electrodes 5 and 10 have been touched, but it is not capable of disambiguating to determine which two of the four possible locations represent the actual touch positions.
In a multi-touch screen, a mutual capacitance measurement may be used to detect simultaneous touches by one or more objects. In the X-Y grid touch screen, for example, mutual capacitance may refer to the capacitive coupling between an X-axis electrode and Y-axis electrode. One set of electrodes on the touch screen may serve as receivers and the electrodes in the other set may serve as transmitters. The driven signal on the transmitter electrode may alter the capacitive measurement taken on the receiver electrode because the two electrodes are coupled through mutual capacitance therebetween. In this manner, the mutual capacitance measurement may not encounter the ambiguity problems associated with self capacitance, as mutual capacitance can effectively address every X-Y proximity relationship (node) on the touch sensor.
More specifically, a multi-touch controller using mutual capacitance measurement may select one electrode in a first set of electrodes to be the receiver. The controller may then measure (one by one) the mutual capacitance for each transmitter electrode in a second set of electrodes. The controller may repeat this process until each of the first set of electrodes has been selected as the receiver. The position of one or more touches may be determined by those mutual capacitance nodes, e.g., nodes 120, experiencing the most significant capacitance change. Projected capacitive touch technology comprising self and mutual capacitive touch detection is more fully described in Technical Bulletin TB3064, entitled “mTouch™ Projected Capacitive Touch Screen Sensing Theory of Operation” by Todd O'Connor, available at www.microchip.com; and commonly owned United States Patent Application Publication No. US 2012/0113047, entitled “Capacitive Touch System Using Both Self and Mutual Capacitance” by Jerry Hanauer; wherein both are hereby incorporated by reference herein for all purposes.
Self and mutual capacitance values may be determined by charging or discharging voltages on the self and mutual capacitances of the electrodes. For example, in the capacitive voltage divider (CVD) method a capacitance value may be determined by first measuring the voltage stored on the electrode capacitor then coupling a discharged know value capacitor in parallel with the electrode capacitor and subsequently measuring the resulting equilibrium voltage, or charging the know value capacitor and coupling it to a discharged electrode capacitor. The CVD method is more fully described in Application Note AN1208, available at www.microchip.com; and a more detailed explanation of the CVD method is presented in commonly owned United States Patent Application Publication No. US 2010/0181180, entitled “Capacitive Touch Sensing using an Internal Capacitor of an Analog-To-Digital Converter (ADC) and a Voltage Reference,” by Dieter Peter; wherein both are hereby incorporated by reference herein for all purposes.
Using a Charge Time Measurement Unit (CTMU), a very accurate capacitance measurement of the electrode capacitance may be obtained by charging or discharging the electrode capacitor with a constant current source then measuring the resulting voltage on electrode capacitor after an accurately measured time period. The CTMU method is more fully described in Microchip application notes AN1250 and AN1375, available at www.microchip.com, and commonly owned U.S. Pat. No. 7,460,441 B2, entitled “Measuring a long time period;” and U.S. Pat. No. 7,764,213 B2, entitled “Current-time digital-to-analog converter,” both by James E. Bartling; wherein all of which are hereby incorporated by reference herein for all purposes.
The charge, Q, on capacitance, C, is directly proportional to the voltage, V, on the capacitance, C, according to the formula: Q=C*V. Therefore, the greater the voltage available to charge or discharge the capacitor, the better the resolution in determining the capacitance values of the electrodes' self and mutual capacitances. In addition, the ability to charge and discharge a capacitance with a higher (greater) voltage also improves the signal-to-noise ratio of the capacitance detection circuit since noise is generally a constant impulse or alternating current (AC) voltage that the electrodes may be shielded from to reduce noise pickup thereon. However, voltages from power sources, e.g., batteries, are being reduced to conserve power by the integrated circuit devices. Therefore the availability of higher voltages is diminishing.